"fill the blank with 了or 着or 过 老板,我写___ 报告___" 1) For the second blank, I'd say only [了] would be correct. 2) For the first blank, all three: [着]， [了]，[过]，can be used correctly if paired with the right context. - [着] is normally used to describe actions that take place (and continue) as you speak - If boss asks if you've written the report (note how it refers to a specific report), and you are in fact working on it at the moment, you can answer with “我写着报告” to mean "I'm currently working on it" (Althought in this case, I'd omit the [了] in the second blank, because it tends to be associated with past/finished actions, which is not the case if you use [着] - The meaning is similar to "我正在写,“ which is what I'd say in this context, just because it sounds more natural - [了] is generally to describe actions that have already taken place, an equivalent to adding -ed at the end of a verb - If boss asks if you've written the report (same specific report), and you've finished it, you can say ”我写了报告了“ to mean "I wrote/finished the report." It is equivalent in meaning to "我写完报告了” - In the two explanations above with [着] and [了], you may be wondering why I noted the "specific report" in both cases. This is because for the third case, [过], it doesn't work well if we are talking about a specific report. Like [了], it also describes past actions, but I would say this one differs in that it is less specific and less conclusive, if that makes sense. I say less specific, because when you answer with "我写过报告了,“ to me, the closest English translation feels like you are saying "I've written reports before," because the use of the [过] is more often used in describing actions that you've done/tried before, in general (hence, reports in general, rather than that specific report), rather than in describing the conclusive completion of a specific action. It would answer the question, "have you written reports before?" Hope it helps!

老板，我写了报告了。也是正确的。但老师不一定算你正确，老师只想你给出”老板，我写过报告了“一个答案。可惜语言往往不只一个答案。

haha, actually, it may depends on different situation. 老板，我写过报告了。 boss, I already wrote the report. 老板，我写着报告了。It‘s like when your boss keep pushing you to finish the report, you say, boss, I am just writing the report now.

老板，我写过报告了。